The invention relates to a process for purifying, particularly for sterilizing or disinfecting fluid media in flow reactors by irradiating with a predetermined minimum irradiation dose of ultraviolet radiation predominantly in the wavelength range between 240 and 320 nm. The invention also relates to apparatus for carrying out such a process including a source of radiation comprising at least one lamp emitting ultraviolet radiation in the wavelength range between 240 and 320 nm, comprising further a flow reactor associated therewith and equipped with means to conduct the medium to be irradiated to and from said reactor and finally comprising monitoring equipment to monitor the ultraviolet radiation passing through said flow reactor.
Processes and apparatus for pufifying, particularly for sterilizing or disinfecting, by the use of ultraviolet rays are employed with advantage over chemical agents for removing from water pathogenic, toxical or otherwisely undesirable bodies sensitive towards ultraviolet radiation. Such undesirable bodies may constitute microorganisms like bacteria, spores, yeasts, or fungi, algae, etc., including viruses or bacteriophages. They also may constitute cancerogenic aromatic compounds, numerous halogen compounds, and most of all chlorine compounds, for instance chlorinated phenols, etc., which will represent impurities burdening the environment. Irradiation of this kind may be employed in connection with the preparation of potable water and it is particularly useful in combination with ion exchange and inverse osmosis plants. Also swimming pool water can be disinfected to the hygienic degree of potable water. Ultraviolet irradiation processes also may be used to treat water circulating for instance in air conditioning units (of the direct air-water contact type) of hospitals. Thus substantially higher degrees of disinfection as required for potable water may be obtained which is a prerequisite of use for instance in connection with opthalmologic preparations or for washing purposes in hospital operating rooms. Further applications will be found for example in breweries and in beverage production, in the food, pharmaceutical and cosmetics industries, photo and electronics industries, in sewage purification, and in the preparation of very high purity seawater for biotechnical purposes.
Photochemical sterilization, disinfection, and detoxication, respectively, follows the known basic principles of photochemical reactions, the validity of which will have to be observed in conducting such reactions in practice. Generally, concentrations of pathogenic and other impurities to be removed by ultraviolet irradiation are relatively low. In practice, the absorption in the medium to be irradiated is therefore determined by other ingredients, the absorption by which competes with that by the microorganisms, etc. In use, it is desirable that the available photon flux should be utilized to the greatest possible extent. Generally chamber depths at which 90% of the incident photons are absorbed will suffice therefor, since doubling of the depth only will result in the further absorption of an additional 9% of the incoming photons. In ultraviolet sterilization techniques depths characterized by 90% absorption are denoted as the "effective depth of penetration". At the wavelength of 254 nm said depth may amount to a multiple of 10 cm (about 4 in.) in particulary pure water, but only to fractions of a millimeter (about 0.04 in.) in milk.
In ultraviolet irradiations in which an extent of 90 to 99% conversion (inactivation) of the microorganisms or, respectively, of the initial impurity present, is achieved there will exist approximately an exponential time dependence as in kinetically analogous photochemical reactions. The aforementioned extent of 90 to 99% conversion will occur within a fraction of the time of that usually required for sterilization or, respectively, detoxication reactions. In such instances the absolute extent of the conversion achieved which will asymptotically approach the initial germ number (numbers of germs per unit volume) may not be of interest. Of much greater interest will be the amount of purified medium of the required degree of purification (as, for instance, 10.sup.-6) obtainable. It will thus be seen that operating at a chamber depth corresponding to 90% absorption, i.e., at the so-called effective depth of penetration, as suggested by photochemical considerations will not produce optimum results. Because of the exponential Lambert law of absorption there will occur an inhomogeneous distribution of purification rates through the depth of irradiation. Since with the presently employed low powered radiation sources the irradiated medium will have a predominantly laminar flow characteristic in passing through the flow reactor, a logarithmic distribution of purification rates will develop within said flow reactor with predominance of the substantially lower purifications at greater distances from the radiation source.
For killing (inactivation) of microorganisms as an example of a purification in the dosage range as required in water disinfection, the simple dose-effect principle will hold approximately. Accordingly, the input concentration of microorganisms (initial number per milliliter) N.sub.o will become reduced by a dose E.multidot.t (E=irradiation intensity; t=period of irradiation) to the concentration of microorganisms N.sub.t at time t in accordance with a sensitivity constant k characteristic for the respective species, to wit: EQU N.sub.t =N.sub.o .multidot.10.sup.-E.multidot.t.multidot.k
With the incoming radiation directed in parallel, the irradiation intensity E itself will become exponentially reduced with increasing depth of the irradiated medium in accordance with the Lambert Beer law of absorption. Altogether, therefore, the following relation will result for the reduction of germ number N.sub.t after irradiation time t: EQU N.sub.t =N.sub.o .multidot.10.sup.-k.multidot.t.multidot.G.multidot.E.multidot.10.spsp.-.al pha.d
wherein .alpha. represents the logarithmic absorption factor of the medium irradiated and d the depth thereof as measured parallel to the path of radiation. With nonparallel incident radiation an additional alteration of radiation intensity will occur in accordance with the geometry of the flow reactor which alteration is accounted for in the aforementioned equation by the corresponding geometry factor G.
In a known photoreactor with approximately parallel incidence of radiation the radiation source is positioned, in a reflector, above the surface of the medium to be irradiated (M. Luckiesh, Applications of Germicidal, Erythemal, and Infrared Energy, Van Nostrand, New York, 1946, pages 257-265; Company brochure LS-179, General Electric Company, "Germicidal Lamps and Applications"). Photoreactors of such a kind may only be used in connection with freely flowing media, however, and not in pressurized systems in which the medium to be irradiated is passed through the photoreactor under pressure. An annular design has been suggested for photoreactors of the latter kind with the radiation source being disposed within the interior annulus space; the radiation source may then be a high pressure mercury lamp (W. Busch, Water Sterilizer "Uster", AEG-Mitteilungen 1936, No. 5, pages 178-181). But also low pressure mercury lamps (K. Wuhrmann, "Disinfection of Water by Means of UV Irradiation", Gas/Wasser/Warme 1960, Vol. 14, pages 100-102) and bundles thereof (P. Ueberall, "Chemicalfree Disinfection of Potable and Service Water by Ultraviolet Rays"; Die Starke 1969, Vol. 21, pages 321-327) have been employed. To compensate for the strong decrease in irradiation intensity because of the Lambert law of absorption and of the photoreactor geometry in annular photoreactors it has been proposed to utilize a number of lamps for the radiation source, each lamp being disposed in a respective reflector, and with the reflectors concentrically surrounding the exterior of the annular flow reactor (German Offenlegungsschrift No. 2119961). Also additional lamps may be used in the interior space (German Offenlegungsschrift 2205598). In the group of photoreactors having radiation sources with radially directed emission there are also included photoreactors having single or multiple radiation sources in an immersion type arrangement in a suitable tank through which the medium to be irradiated flows (L. Grun, M. Pitz; "UV Rays in Jet Chambers and Air Passages of Air Conditioning Equipment in Hospitals", Zbl. fur Hygiene, I. Abteilung Orig. 1974, Vol. B159, pages 50-60).
Although effective depths of penetration with 90% absorption are known for many media, known photoreactors generally provide for depths of just a fraction thereof. For disinfecting potable water on sea-going vessels there even exists a regulation according to which the depth of the medium to be irradiated is not permitted to exceed 7.62 cm (3 in.; Department of Health, Education and Welfare, Public Health Service; Division of Environmental Engineering and Food Protection; "Policy Statement on Use of the UV Process for Disinfection of Water"; Apr. 1, 1966). While such a requirement may be significant for safety reasons, it results in loss of the opportunity for economic disinfection in many cases of media having high transmission factors since a substantial portion of the photon energy entering the medium is not actually utilized and is lost in the photoreactor walls. Attempts to salvage the unused portion by providing reflective walls have not proven particularly effective.
On irradiating (parallel incidence) at a depth of 90% absorption at such a high dosage that disinfection within the first layer encompassing 10% absorption reaches values of at least 10.sup.-10 an inhomogeneity of disinfection degrees in accordance with the foregoing equation will result covering the range of 10.sup.-9 in the closest layer to 10.sup.-1 in the most remote layer. An average value of the order of 10.sup.-2 will then be obtained for the degree of disinfection which is of little satisfaction considering that the theoretically obtainable degree of disinfection will be in the order of 10.sup.-4 as calculated with the assumption of a non-logarithmically decreasing mean radiation intensity.
The object to be achieved by the present invention accordingly is to provide a process and apparatus which will permit optimum utilization of the ultraviolet radiation as emitted from the radiation source at as high a throughput of the fluid medium as possible.
This object is achieved by the present invention in passing the medium through separate irradiation chambers of a flow reactor subdivided at right angles with respect to the general flow direction (path) of the radiation and by having portions of the radiation incident in the first irradiation chamber, i.e., the chamber closest to the radiation source, pass into at least the directly adjoining irradiation chamber.
The invention starts from recognizing that by subdividing the photoreactor the depth of each irradiation chamber may be selected in such a way that the variation in radiation intensity through the depth of the respective chamber does not too unfavorably affect the economy of the irradiation process. Thereby the distribution of the degree of disinfection through each respective irradiation chamber will become less inhomogeneous. Quadruple or quintuple subdivision of a depth providing 90% overall absorption may result in differences between the degree of disinfection within each irradiation chamber of less than 3 orders of magnitude, while such differences may encompass more than 8 orders of magnitude in a nondivided photoreactor. The principle of the invention is thus seen to be based on adjusting the efficiency of the photoreactor, which passes through an optimum with increasing depth and then strongly decreases, in such a way as to operate at a depth of only fractional absorption, utilizing the photons issuing therefrom in adjoining chambers of similar or the same depth also providing for only fractional absorption. The favorable effect as obtained by the subdivision is widely independent of the irradiation geometry of the respective photoreactor. Such effect will be achieved as well in photoreactors with a radiation source of the immersion type arrangement as in annular photoreactors with the radiation source disposed in the interior space and/or externally thereof. The effect will also be realized in photoreactors of the type in which the radiation source is positioned above the surface of the medium.
It has been found on closer analysis that the economy of the irradiation process is particularly strongly affected in the negative by the inhomogeneity of the disinfection degree in those layers of the irradiated medium which are exposed to the highest irradiation intensity. For utilizing, on the one hand, as much as possible of the high radiation intensity prevailing immediately adjacently to the radiation source which is specifically efficient in the disinfection and, on the other hand, for suffering as little loss as possible on this favorable effect by the inhomogeneity in the distribution of degrees of disinfection, absorption in the irradiation chamber immediately adjacent to the radiation source should not exceed 60% of the incoming radiation.
Advantageously at least 50% of the radiation entering into the medium present in the irradiation chamber immediately adjacent to the radiation source will enter into the directly adjoining irradiation chamber (that is, not more than 50% of the incident radiation being absorbed in said medium present in said irradiation chamber immediately adjacent to said radiation source); in a flow reactor having up to 5 irradiation chambers not more than (1-0.5.sup.n).multidot.100 percent of the totally incident radiation should become absorbed, wherein n represents the number of irradiation chambers. It is not necessary, however, that the incident radiation become attenuated by the same fraction in each respective irradiation chamber. In keeping with the foregoing discussion the efficiency of the purification or disinfection, respectively, is determined by the radiation intensity gradient existing between incident and emergent radiation in each respective irradiation chamber. This will hold for each single irradiation chamber in a multichamber photoreactor so that in the case of, for instance, two irradiation chambers the total absorption of the incident radiation should not exceed 75% to keep said gradient sufficiently small with respect to each single irradiation chamber and to maintain as high an overall efficiency as possible.
An effort has already been made, in connection with a single chamber photoreactor, to reduce the detrimental effects originating from the radiation intensity gradient within the irradiation chamber by intensely mixing the medium while it is present within the single chamber (French Patent Specification No. 1,560,780; German Offenlegungsschrift No. 1937126). However, even at very high turbulences an ideal mixture in which all particles of the medium would be exposed to the same mean radiation intensity cannot be attained. Still, not even such ideal mixture would be capable of removing the effect of the radiation intensity gradient within the medium since the mean radiation intensity will decrease with increasing depth. As will be shown by detailed calculation, the gradient will become effective, to a degree acceptable for practical purposes, at irradiation depths at which not more than 60%, and preferably 50%, of the incident radiation is absorbed. Thus the efficiency of purification or disinfection, respectively, will be considerably higher in a two-chamber photoreactor as compared to a single-chamber photoreactor of the same overall depth. A further advantage in the multiple chamber photoreactor arises from removal of the effect of the flow characteristics of the medium within the irradiation chambers on the purification or disinfection, respectively, under such conditions. Specific means for generating turbulent flow through the irradiation chamber, therefore, may be dispensed with in multiple chamber photoreactors.
To increase the degree of disinfection it may be advantageous to introduce an oxidizing agent into the medium before or during irradiation. The oxidizing agent may be oxygen, ozone, halogen, or some hypohalogenite, for example. Thus not only oxidative decomposition of impurities present in the medium will be furthered, but also the disinfection will be favorably influenced by additional secondary bactericidal effects.
Sensitivities of microorganisms towards ultraviolet radiation differ very much; for instance, that of fungi or algae is lower by two orders of magnitude than the sensitivity of bacteria. In the use of flow reactors for disinfection there will thus result a wide dosage range which may not be covered in its entirety simply by increasing the flux of radiation emitted from the radiation source and/or by reducing the throughput of the medium being irradiated. According to the invention it is therefore provided that at least a portion of the flow of the irradiated medium after passing the flow reactor is reintroduced into the same. Thus the medium being irradiated is fed a number of times through the reactor and will become irradiated with a corresponding multiple of the single passage dose. Such a procedure is also recommended in cases in which varying amounts of the disinfected medium are withdrawn from the ultraviolet disinfection unit.
According to the process of the invention the medium is exposed to ultraviolet radiation in the wavelength range between 260 and 280 nm. Ultraviolet radiation of this wavelength range is particularly effective in photodisinfection because microorganisms show maximum sensitivity in this range (L. J. Buttolph, "Practical Application and Source of Ultraviolet Energy"; Radiation Biology, McGraw Hill, New York 1955, Vol. 2, pages 41-93). Irradiation within that wavelength range also prevents photochemical formation of precipitates from media containing iron or manganese which precipitations occur on irradiation with low pressure mercury lamps at 254 nm. Another particular advantage of irradiating in the 260 to 280 nm wavelength range resides in the strong decrease in the absorption of iron or manganese containing impurities within this range which therefore act much less as a radiation filter diminishing the efficiency of the radiation in the photodisinfection than in the wavelength range as emitted by low pressure mercury lamps.
In accordance with the process of the invention the medium is advantageously passed successively through the irradiation chambers of the flow reactor. Thereby, as explained above, the efficiency of the purification or disinfection, respectively, will be considerably increased. the flow rates within each of the irradiation chambers of the multiple chamber flow reactor being increased over the flow rate in a single chamber photoreactor. Flow short-circuits occurring in single chamber photoreactors at high chamber depths and concurrent low flow rates will thus be avoided because of the increased flow rates at smaller cross-sections of the irradiation chamber. Such flow short-circuits will result in the formation of highly differentiated flow rates within the radiation field of the single chamber photoreactor and such differentiation will tend to become more pronounced at decreased flow rates and may result in the overall result of the irradiation being questionable. Additionally it is recommended to operate the multiple chamber photoreactor at a flow rate at or above the limit of turbulence of the medium flowing therethrough. Thus not only the formation of precipitates from the irradiated medium will be effectively suppressed in the mutliple chamber photoreactor but furthermore the heat transfer from the radiation source to the medium flowing through that chamber which is immediately adjacent to the radiation source will be particularly favorable so that overheating is avoided.
The considerable increase in the efficiency of the flow reactor by the described subdividing is not subject to the medium being fed successively through all of the irradiation chambers of the flow reactors. To a significant extent this increase is an intrinsic property of the multiple chamber photoreactor itself. To wit, if the medium is conducted in parallel through the irradiation chambers the flow rate in each single chamber may be adjusted such that the same minimum dose is applied in each of said irradiation chambers so that the same degrees of disinfection will be obtained and the portions of the medium flowing at different flow rates may be recombined after leaving the irradiation chambers. While parallel directions of flow are more elaborate with respect to apparatus and equipment, such may be advantageous if simultaneous irradiation of different media is desired.
The apparatus according to the invention is characterized by: the flow reactor is subdivided into separate irradiation chambers by windows extending normally with respect to the general direction of irradiation and made of material transparent for the ultraviolet radiation; the radiation incident into the medium present in the second irradiation chamber from the radiation source amounts to fractions of the radiation entering into the irradiation chamber next to said radiation source; and the monitoring equipment for maintaining the predetermined minimum irradiation dose comprises medium flow control means connected to the input or the outlet of said flow reactor. The fraction of the radiation incident in the second irradiation chamber from the radiation source should amount to at least 50% of the radiation entering into the irradiation chamber next to said radiation source and the absorption in the last said irradiation chamber should not exceed 50% with the total absorption in a flow reactor having up to 5 irradiation chambers not exceeding (1-0.multidot.5.sup.n).multidot.100 percent of the overall incident radiation, n representing the number of irradiation chambers.
In the field of hospital hygiene disinfection units are known which operate in such a way as to circulate the medium to be disinfected, like the wash water of air conditioners (of the type wherein there is direct contact between the air and water) through a tank into which a radiation source comprising one or more lamps is inserted in an immersion type arrangement with each lamp being fitted into an enveloping tube. Such arrangements have unfavorable flow conditions resulting in portions of the water in the tank receiving much higher radiation doses than required while the greater portion thereof will remain exposed to less than the required dose. Therefore, there is the danger that such water when contacting the air being conditioned will give off germs to that air. According to the present invention, the enveloping tube for each lamp is surrounded by at least one silica glass tube so as to form at least one inner irradiation chamber and that said inner irradiation chambers are connected in common either to the input or to the outlet of the flow reactor. Contrary to the known arrangements such apparatus ensures that the irradiation occurs at the minimum dose required independent of the direction of flow through the inner irradiation chambers. If the inner irradiation chambers are then connected to the input, irradiation in the presence of oxygen or other gases will be facilitated; if connected to the flow reactor outlet optimally disinfected water will be delivered at the spray nozzle of the air conditioner.